Earth leakage protection of apparatus and machines



July 3, 1934. w U w 1,965,485

EARTH LEAKAGE PROTECTION OF APPARATU$ AND MACHINES Filed Dec. 9, 1529 I05 i 0 F 0 50 200 750 Va/f l Patented July 3, 1934 UNITED STATES PATENTOFFICE EARTH LEAKAGE PROTECTION OF APPARATUS AND MACHINES ApplicationDecember 9, 1929, Serial No. 412,893 In Germany March 29, 1926 3 Claims.

My invention relates to protective appliances for electric plants.

It is an object of my invention to provide a protective appliance for aplant with a power 5 transformator intermediate 2. system and anelectric apparatus, for instance, a generator.

To this end, I provide a voltage transformer which is connected to thezero point of the apparatus or generator and to earth, and has a relayin its secondary circuit. The relay which may be of the amperemetric orwattmetric type, has its coil (or, in the case of a wattmetric relay,its voltage coil) shunted by a ballast resistor which is directlyconnected in parallel to the secondary of the voltage transformer.

Ballast or Nernst resistors are old in the art and consist of a pureiron wire suspended in hydrogen gas.

It has already been proposed in plants in which the apparatus to beprotected operates directly on the system, and not on an intermediatepower transformer, to provide an earthed voltage transformer with arelay and a ballast resistor in the secondary circuit of the voltagetransformer.

This ballast resistor, however, is connected in series to the coil ofthe relay, and not in shunt as in the present invention.

Conditions in a plant without an intermediate power transformer arequite different from those in a plant with such a transformer, to whichthe present invention relates.

In plants without a power transformer the leakage current is alwayssufficient for operating a safety appliance while in plants with a powertransformer the capacity of the phases to earth is so small that it isnecessary first and foremost to generate a leakage current of therequired intensity. The leakage current was generated heretofore in avoltage transformer in combination with an invariable resistor.

According to my invention the invariable resistor is replaced by aballast resistor arranged as described. I may combine a shunting ballastresistor with the series ballast resistor referred to above, but myinvention resides in providing the shunting ballast resistor.

In the accompanying drawing Figure 1 is a curve showing the relationbetween the current and the terminal voltage in ballast resistors; Figs.2, 3 and 4 are diagrams of connections which may be employed accordingto my invention.

The diagram illustrated in Fig. 1 in which current is plotted againstvoltage shows the well known property of ballast resistors, i. e. thatthe current flowing through the resistor rises at first rapidly with theincrease of the terminal voltage but afterwards remains nearly constantalthough the terminal voltage is further increased. By providing a,ballast resistance which shunts the coil, or a coil, of the relay, theearth leakage current will be limited approximately to the value whichis required for the operation of the relay. This arrangement has thegreat advantage that burning which for instance takes place in case ofan earth leakage within a generator is greatly 55 limited. In many casesit is possible to continue to work for hours with the defective machineor with the apparatus exposed to earth leakage without increase in theburning. It has been found that the earth leakage current should be 7limited to about 5 amps, as against the usual intensity of about 20amps. By the shunting ballast resistor according to my invention theearth leakage current is limited to about A, of this amount and burningin the machine to be protected by my appliance is prevented or does notspread. In such a case with any organic insulating materials the layerof carbon produced by the action of the voltaic arc is sufficient todischarge the earth leakage current.

A plant with an amperemetric relay in the secondary circuit of thevoltage transformer and a ballast resistor shunting the coil of therelay, is illustrated in Fig. 2 where T is the power transformer and Gis an apparatus, such as a generator, which is connected through thepower transformer T to high voltage system N. S is a voltage transformerwhich is connected to the zero point of the apparatus or generator G andto earth, and R is an amperemetric relay with its coil in the secondarycircuit of voltage transformer S. B is a ballast resistor which shuntsthe coil of relay R and is directly connected in parallel to thesecondary of the voltage transformer S. L is another ballast resistorwhich is connected to the secondary in series and may, or may not, beprovided. The resistors B and L are so designed that the earth leakagecurrent will be limited approximately to the value which corresponds tothe operating current of the l00 amperemetric relay R and never exceeds5 amperes. Obviously instead of using individual ballast resistances Bor L for instance I may subdivide these resistors into a plurality ofresistors of equal action and even form groups of those partialresistors connected together without departing from the gist of theinvention. With a ballast resistor, or resistors, according to myinvention it is always possible to fulfill the condition that theequivalent resistance in place of the primary side of the voltagetransformer S becomes small in the case of an earth leakage on the highvoltage side of the power transformer T, but becomes large in the caseof an earth leakage on the low voltage side of the power transformer Tor on the generator side.

The arrangement according to the diagram shown in Fig. 2 is preferablyemployed if the machines or apparatus to be protected work on a system Nof comparatively small extension. If the said machines and apparatuswork on a system of a large extension it is only necessary to replacethe amperemetric relay R by a wattmetric relay R as shown in Figs. 3 and4 having two co-operating exciting windings or coils one of which is thevoltage coil whilst the other is the current coil.

Figs. 3 and 4 show that the voltage coil of the wattmetric relay R isconnected to the secondary of transformer S in the same manner as thesingle coil of the amperemetric relay R in the arrangement illustratedin Fig. 2, and shunted by the ballast resistor B. L is the seriesballast resistor.

According to Fig. 3 the current coil of the wattmetric relay R. isconnected to the secondary side W of a so called integrating currenttransformer E the iron core of which is indicated in dotted lines. 1ecurrent flowing through W corresponds to the sum of the currents whichflow to or from the generator G.

Instead of using one special transformer E integrating the currentswhich flow to or from the generator G standard current transformers thesecondary windings of which are connected in parallel may also be usedas illustrated in Fig. 4 showing four standard current transformers E1,E2, E3 and E4 with paralleled secondary windings W1, W2, W3 and W4respectively. The current coil of the wattmetric relay R is connected tothese windings. The voltage coil of this relay is shunted by theinvariable resistance X so that the terminal voltage is determined bythe current flowing through X in consequence whereof the said terminalvoltage is maintained throughout its full value independent of thevoltage between the neutral or zero point of the generator G and earth.

I claim:

1. In an electric plant, an electric apparatus, a system, a powertransformer intermediate said system and said apparatus, and aprotective appliance including: a voltage transformer connected to theZero point of said apparatus and to earth, a relay in the secondarycircuit of said voltage transformer. a coil in said relay, a ballastresistor connected in parallel to said coil and directly connected inparallel to the secondary of said voltage transformer, and anotherballast resistor in series with said coil.

2. In an electric plant, an electric apparatus, a system, a powertransrormer intermediate said system and said apparatus, and aprotective appliance including: a voltage transformer connected to theZero point of said apparatus and to earth, a wattrnetric relay in thesecondary circuit of said voltage transformer, a voltage coil in saidrelay, a ballast resistor connected in parallel to said coil anddirectly connected in parallel to the secondary of said voltagetransformer, and another ballast resistor in series with said coil.

3. In an electric plant, an electric apparatus,

a system, a power transformer intermediate said system and saidapparatus, and a protective appliance including: a voltage transformerconnected to the zero point of said apparatus and to earth, awattrnetric relay in the secondary circuit of said voltage transformer,a voltage coil in said relay, a ballast resistor connected in parallelto said coil and directly connected in parallel to the secondary of saidvoltage transformer, another ballast resistor in series with said coil,and an invariable resistance connected in parallel to said voltage coil.WALTER BtiTow.

